(a) Field of the Invention
The present invention relates to a compact, large-aperture objective lens and a compact imaging optical system to be used in the optical information recording and readout device.
(b) Description of the Prior Art
The optical system used in the optical information recording and readout device (hereinafter referred to as the optical memory device) has such a composition as shown in FIG. 1. Speaking concretely of the component members with reference to FIG. 1, the reference numeral 1 represents an information recording medium such as an optical disk, the reference numeral 2 designates an objective lens, the reference numeral 3 denotes a collimator lens and the reference numeral 4 represents a light source. The imaging optical system for the optical memory device having the composition described above is used for making parallel the rays emitted from the light source with the collimator lens and focusing the rays onto the information recording surface of the information recording medium 1 with the objective lens 2.
A condenser lens (objective lens) to be used in this optical system should desirably have not only favorable imaging performance but also a compact, light-weight design since it must be shifted speedily for automatic focusing and tracking.
As objective lenses for the optical memory device each consisting of a single-element lens having aspherical surface, there are known the objective lenses disclosed by Japanese Unexamined Published Patent Application Nos. 76512/57, 23313/59 and 26717/59. These objective lenses make it possible to design compact, light-weight optical memory devices manufacturable at low costs.
In order to make an objective lens for optical memory device smaller, it is necessary to minimize the entrance pupil of the lens. The entrance pupil of a lens is given as a product of the focal length multiplied by the numerical aperture thereof and, it is necessary to shorten focal length for making compacter the objective lens for optical memory device since it has a numerical aperture larger than 0.45.
Out of the conventional examples mentioned above, the objective lens disclosed by Japanese Unexamined Published Patent Application No. 76512/57 has a large lean thickness d and a ratio d/f on the order of 0.7 between the thickness and focal length thereof. Accordingly, the working distance of the objective lens is shortened when the focal length thereof is shortened.
Further, aberrations are not corrected sufficiently favorably in the objective lens disclosed by Japanese Unexamined Published Patent Application No. 26717/59.
Furthermore, the objective lens disclosed by Japanese Unexamined Published Patent Application No. 56314/61 is a finite optical system (an optical system focusing an image of a light source on a disk located at a finite distance) and permits omitting the collimator lens in the imaging optical system for the optical memory device. Therefore, the objective lens makes it possible to simplify the composition of the imaging optical system as a whole and reduce manufacturing cost thereof. However, since the objective lens is a finite optical system, it has a defect that it causes variation of magnification and degradation of imaging performance when the lens follows fluctuations of the optical disk and so on. In order to correct this defect, it is necessary to reserve a long distance between a semiconductor laser source and a disk, thereby enlarging the optical memory device and lowering the ratio of the rays effectively used for imaging out of the rays omitted from the laser source (utilization efficiency of laser).
Moreover, as conventional examples of the optical systems for the optical memory device, there are known the optical systems disclosed by Japanese Unexamined Published Patent Application No. 43842/62.
The latter optical system is so adapted as to permit enhancing utilization efficiency of laser and reducing the degradation of performance caused when the objective lens is shifted in the direction along the optical axis and the tracking direction by interposing a coupling lens between a semiconductor laser source and the objective lens. However, since the objective lens is a finite system, the objective lens has an effective aperture (1+.beta.) times as large as that of an objective lens of the infinite system for obtaining the same numerical aperture (.beta. represents imaging magnification of the imaging optical system), thereby enlarging diameter of the lens and making it impossible to make compact the optical memory device. Further, the reduction of performance degradation is not sufficient. Especially, performance is remarkably degraded by shifting the objective lens in the direction along the optical axis.